A back-illuminated solid-state image pickup apparatus according to the related art in which transistors and metal wires are arranged on a first main surface (front surface) of a semiconductor substrate and a second main surface (back surface) opposite to the front surface is illuminated with light has been proposed to provide a highly sensitive solid-state image pickup apparatus.
PTL 1 describes a back-illuminated solid-state image pickup apparatus having photo detectors stacked in the depth direction of a semiconductor substrate. The solid-state image pickup apparatus described in PTL 1 detects, at the respective photo detectors, light in wavelength bands corresponding to the respective depths of the photo detectors, employing the fact that optical absorption coefficient of the semiconductor substrate material is wavelength dependent. For example, when three photo detectors are stacked, the photo detector closest to the incident surface mainly detects blue light, the photo detector in the middle mainly detects green light, and the photo detector farthest from the incident surface mainly detects red light.
In the solid-state image pickup apparatus described in PTL 1, the photo detectors each have an impurity diffusion region extending in the depth direction for electrically connecting the photo detectors with circuits on the front surface.
With the configuration described in PTL 1, light enters the impurity diffusion regions extending in the depth direction of the semiconductor substrate. For example, when incident light is photo-electrically converted at a position deep in an impurity diffusion region corresponding to a blue photo detector, the generated carriers are accumulated as signal charges for blue. However, these carriers should actually be accumulated as signal carriers for green and red. By accumulating the carriers as signal carriers for blue, the color separation characteristic is aggravated, causing noise.
A front incident solid-state image pickup apparatus includes light-shielding structures, such as transistors and wires, on the light incident surface. In contrast, with a back incident solid-state image pickup apparatus, since transistors and wires are not required on the incident surface, light is incident on the entire back surface. Therefore, aggravation of the color separation characteristic becomes more obvious.
The present invention has been conceived in light of the problem described above and provides a back-illuminated solid-state image pickup apparatus having an improved color separation characteristic.